The present invention relates to an image fixing device whose main component is a fixing belt, and to an image forming apparatus such as a copier and a printer structured so that a variation of a path of the fixing belt in the image fixing device is controlled following a fixing processing motion.
An image forming apparatus having the image fixing device (hereinafter, simply called the fixing device) structured in such a manner that a sheet on which a toner image is carried, is fed between the fixing belt which is supported by a plurality of rotatable support rollers and heated by an appropriate heat applying source, and a pressure roller provided so that it is rotated while being pressure-contacted with the fixing belt, and the pressing and heating actions are given to it, and the toner image is fixed onto the sheet, is well known.
As one embodiment of the fixing device as described above, there is considered a structure by which, for example, two pressure rollers are oppositely arranged to each other with the fixing belt between them, and in the advancing direction of the fixing belt, an axial center is slightly shifted, and at the time of the fixing processing, a relative position of both pressure rollers is changed, and the fixing belt is forcibly bent and supported, and to the sheet passing between the fixing belt and the pressure roller, its contact area contributing to the fixing is increased.
Although such the enlargement of the fixing nip width makes the thermal efficiency enhance, and the stable fixing processing possible, according to circumstances, in order to obtain the necessary length for the curvature formation of the fixing belt at the time of the fixing, there is a case in which it is necessary that a position of any support roller is moved.
At that case, it is also considered that the support roller whose position is moved, is a structure in which it includes internally (hereinafter, also called internally contained) the heat applying source.
The object of the present invention is to provide an image fixing device structured in such a manner that the heat applying source is housed in one of support rollers around which the fixing belt is trained as described above, and accompanied by the forcible variation (the same meaning as the above-described forcible curvature formation) of the traveling path of the fixing belt, the position of the support roller in which the heat applying source is housed, can be safely moved, and to provide an image forming apparatus which can control so that a wide nip portion is structured accompanied by the fixing processing, and the enhancement of the thermal efficiency can be attained.
The purpose of the present invention can be attained by the following structures.
(1) An image fixing apparatus which is characterized in that, by training a fixing belt around a plurality of support rollers including one support roller housing a heat applying source, and by changing a relative position of a pair of pressure rollers which are oppositely arranged with the fixing belt interposed between them, to a pressure position, a traveling path of the fixing belt is changed, and through the fixing belt whose path is changed, the heat applying source and the support roller housing the heat applying source are structured to be integrally movable.
(2) An image fixing apparatus which is characterized by having: a fixing belt trained around a plurality of support rollers including a rotatable support roller housing the heat applying source; a pressure roller which is selectively positioned at a pressure position to which a traveling path of the fixing belt is changed, by pressing the fixing belt; and a support member whose drive source is the fixing belt whose traveling path is varied, and by which the heat applying source, the support roller housing the heat applying source and at least one portion of a casing to shield the support roller from the periphery thereof, are supported so as to be integrally movable.
(3) An image forming apparatus which is characterized in that it has the image fixing apparatus described in above structure (1) or (2), and has a control means for controlling a movement to a pressure position of the pressure roller onto the fixing belt and a pressure release, accompanied by a fixing processing operation.